Atomic force microscopy measurements of contact resistance and current-dependent stiction

ABSTRACT

A modified atomic force microscope (AFM) is used to perform contact resistance and/or current-dependent stiction measurements for conductive thin films at controlled values of applied force. The measurements are preferably performed under conditions approximating the operation of the thin films as electrodes in microswitch array fingerprint sensors. A first, planar thin film is contacted with a second, curved thin film deposited over a round ball having a diameter of a few microns to a few tens of microns. The second film is preferably a coating deposited over the ball and over the arm controlling the ball motion. The coating deposited over the arm provides an electrically conductive path to the contact surface of the ball.

RELATED APPLICATION DATA

[0001] This is a divisional of application Ser. No. 10/202,439 filedJul. 23, 2002 and is related to U.S. patent application Ser. No.09/571,765 filed May 18, 2000 entitled “Method and Apparatus forPressure Sensing,” and U.S. patent application Ser. No. 10/038,505 filedDec. 20, 2001 entitled “Fingerprint Sensors using Membrane SwitchArrays.”

FIELD OF THE INVENTION

[0002] The invention relates to systems and methods for measuringcontact resistance and/or stiction, and in particular to methods ofevaluating contact materials for microswitches such as fingerprintsensor switches.

BACKGROUND OF THE INVENTION

[0003] The fingerprint sensing industry uses several differentconventional technologies to capture images of an individual'sfingerprints. Two prominent technologies are optical-based sensors andcapacitance-based sensors. In a typical optical sensor, a light source,lenses and a prism are used to image the ridges and valleys on afingerprint, based on differences in the reflected light from thefeatures. Conventional capacitance sensors include two-dimensional arrayof capacitors defined on a silicon chip, and fabricated by semiconductorCMOS processing. The individual sensors on the chip form one plate ofthe parallel plate capacitor, while the finger itself, when placed onthe array, acts as the second plate for the various localized sensors.Upon contact with the array of sensors, the individual distance fromeach sensor to the corresponding point on the skin above the sensor ismeasured using capacitive techniques. The difference in distance to skinat the ridges and valleys of a fingerprint identifies the fingerprint.

[0004] Capacitive and optical sensors can be sensitive to oils or greaseon the finger and to the presence or absence of moisture on the finger.In addition, the ambient temperature can affect these sensors at thetime of sensing. Under hot or cold conditions, capacitive sensors canprovide erroneous readings. Finally, most sensors have abrasionresistant coatings. The thickness of the protective coating can affectthe measurements. The combined effect of these variables can result indistorted fingerprint images. Finally, in the case of silicon chip basedfingerprint sensors, the placement of the finger directly onto thesilicon increases the risk of electrostatic discharge and damage to thesensor.

[0005] The above-referenced U.S. patent application Ser. Nos. 09/571,765and 10/038,505 describe systems and methods for performing texture (e.g.fingerprint) measurements using switch arrays. A fingerprint sensor forperforming such measurements can include tens of thousands of miniatureswitches arranged in an x-y array. Each switch includes a lowerelectrode, and an upper electrode disposed over the lower electrode.Depending on the force applied on the switch, the upper electrode can beseparated from the lower electrode, or can establish electrical contactwith the lower electrode. The state (open or closed) of each switchindicates whether a fingerprint ridge or valley is positioned above thatswitch. A map showing the distribution of switch states over the sensorarea can thus be used to identify a fingerprint positioned over thesensor.

[0006] In a representative implementation of such a fingerprint sensor,each switch extends over a few tens of μm along the x- and y-directions,and is capable of changing state upon the application of a loadcorresponding to a few mg of mass. Such a switch can include upper andlower electrodes formed by thin films having thicknesses on the order oftenths of micron to a few microns. The performance of such a switch candepend to a significant extent on the properties of the thin filmsdefining the electrodes. Systems and methods for systematicallyevaluating such thin films could be of great benefit to the design ofmore accurate and reliable switches for texture sensing and otherapplications.

SUMMARY OF THE INVENTION

[0007] The present invention provides systems and methods for performingmeasurements that are useful in determining materials that are useablefor various applications and particularly contact materials useable forswitches of different types, and more particularly for microswitches,using an atomic force microscopy measurement. A measurement ormeasurements, for instance of contact resistance between a pair ofopposing thin films, is performed at a first controlled applied forcevalue between the thin films. Each of the thin films is formed by amaterial being considered for use in the application, switch ormicroswitch of interest. A subsequent atomic force microscopymeasurement is used to obtain a current-dependent stiction force valuebetween the thin films. The contact resistance, the stiction force, orother measurement values, either separately or in combination, areemployed to determine materials that are suitable for the application,and particularly contact materials that will contact each other inswitches or microswitches. Of particular interest is measuringcharacteristics of thin film materials.

[0008] In one aspect of the invention to perform the measurements, afirst film of a first contact material is disposed on a planarsubstrate. A second film of a second contact material is disposed on arounded piece. The radius of curvature of the rounded piece along acontact surface between the first film and the second film is preferablyhigher than 10 μm and lower than 100 μm. Measurements between the firstfilm and the second film are then performed at a controlled appliedforce value between the first thin film and the second thin film,preferably not exceeding 1 milliNewton (mN).

[0009] Other aspects and advantages of the present invention will becomeapparent hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The foregoing aspects and advantages of the present inventionwill become better understood upon reading the following detaileddescription and upon reference to the drawings where:

[0011]FIG. 1 is a schematic diagram of a modified atomic forcemicroscopy apparatus for performing contact resistance and/or stictionmeasurements, according to the preferred embodiment of the presentinvention.

[0012]FIG. 1-B is a schematic diagram of a contact resistancemeasurement circuit according to an embodiment of the present invention.

[0013]FIG. 2 shows measured and computed contact resistance values forseveral thin films, according to an embodiment of the present invention.

[0014] FIGS. 3-A and 3-B show computed contact resistance values forRu—Mo contact surfaces, according to an embodiment of the presentinvention.

[0015]FIG. 4 shows stiction force values for two Au—Au contact surfaces,according to an embodiment of the present invention.

[0016] FIGS. 5-A and 5-B are side view illustrations of the depositionof a conductive film on a movable member, before and after thedeposition, respectively, according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] In the following description, it is understood that each recitedelement or structure can be formed by or be part of a monolithicstructure, or be formed from multiple distinct structures. Anelectrically-dependent or current-dependent stiction force value is avalue of a stiction force between two surfaces measured after or duringa passage of electrical current through the-contact interface betweenthe two surfaces. Unless explicitly specified otherwise, a thin film isunderstood to be a film having a thickness of less than about 1 micron.The term “material” is understood to encompass composite and/or alloystructures comprising multiple chemical elements or compositions. Theterms “upper” and “lower” are used to describe relative positions, anddo not necessarily refer to the direction of gravity. A set of elementsis understood to include one or more elements. A plurality of elementsis understood to include two or more elements. Any recitation of anelement is understood to refer to at least one element. Unless specifiedotherwise, any recitation of a first element and a second element (e.g.first and second applied forces) is understood to allow for a firstelement equal to the second element. For example, a first material of apair of contact materials need not be necessarily different from asecond material of the pair of contact materials.

[0018] The following description illustrates aspects of the invention byway of example and not necessarily by way of limitation.

[0019] The design of microswitches used for fingerprint recognition andother applications would benefit from systematic methods of evaluatingthe thin metal films that define the contact surfaces, at contact forcevalues and electrical current conditions corresponding to the operatingconditions of the switches. Parameters of particular interest for theoperation of such switches are contact resistance and stiction.Selecting electrode materials with low contact resistance and stictionforces facilitates discriminating between the open and closed states ofthe switches, and reduces the likelihood that switches remainpermanently closed. Microswitch applications where such contact materialevaluation techniques would be of benefit include communications, powerelectronics, indicators and instrumentation, industrial automation,automotive applications, and applications involving extreme operatingconditions, such as space and defense applications.

[0020] The importance of providing low contact resistance and lowcontact stiction increases as the microswitches are miniaturized toever-smaller sizes, and operate at smaller forces. For micro-switches,the force with which surfaces mate can be in the microNewton tomilliNewton range. For clean surfaces in asperity contact, theasperities along the surfaces constrain the flow of current, thuscreating contact resistance. Contamination along the contact surfacescan introduce an additional resistance at the contact interface.Current-dependent stiction is the force with which two mating surfacesare held, due to microwelding, after the application of force andcurrent. Stiction can occur even in the absence of current flow, due tomechanical alloying resulting from applied force. The stiction effect issignificantly more pronounced with the passage of current, as thepassing current induces heating of asperities along the contactinterface and promotes micro-welding. If the stiction force exceeds therestoring force of the micro-switch, the micro-switch can becomepermanently closed or stuck, and can thus stop functioning. As switchesare miniaturized, their restoring forces often decrease, and theirvulnerability to stiction-induced failure increases.

[0021] Both contact resistance and stiction forces are stronglydependent on the geometry of contact. To simulate real contacts, theevaluation method ideally reproduce the real micro-switch geometry. Forfingerprint sensors, for example, where contact is between a flatsurface (typically the flat lower electrode) and a slightly roundedsurface (typically the flexing upper electrode), and the contact area issub-micron in spatial dimension, sharp probes or macroscopically largeprobes would not yield accurate results reflecting the operation of themicroswitch. The preferred embodiment of this invention aims to betterreproduce the geometry of real microswitches, as well as provideaccurate and convenient ways of measuring contact resistance andstiction.

[0022] Consider a typical load applied by an individual's finger on amicroswitch array sensor, as described in further detail in theabove-referenced U.S. patent application Ser. No. 10/038,505. Consideran applied load in the range of 100-500 grams, a fingerprintapproximately 15 mm×15 mm in general diameter, and an array of switcheswith total dimensions of 15 mm×15 mm. The spacing between typicalfingerprint ridges is on the order of 400 μm. If the switches are placed50 μm apart on a two dimensional x-y grid, an array of on the order of300×300 switches would be suitable for covering a sensor surface area of15 mm×15 mm. There are a total of 90,000 sensors in such an array, andthe applied load from the fingertip can be assumed for simplicity to bedistributed over these 90,000 sensors. As a first order approximation,one can assume that the area of the ridges is equal to that of thevalleys. Thus, approximately 45,000 sensors bear the applied load. Ifone conservatively assumes an applied load of 90 grams from thefingerprint, then each cell bears an approximate load of about 2 mg.Such a load corresponds approximately to an applied force on the orderof tens of microNewtons (μN), e.g. 10-30 μN.

[0023]FIG. 1 shows a schematic side view of a contact resistance andstiction measurement apparatus 20 according to the preferred embodimentof the present invention. Apparatus 20 is preferably a modified atomicforce microscope (AFM). Apparatus 20 can be made by performingmodifications as described below on a commercially available AFM, suchas a Veeco/Digital Instruments Dimension-series AFM. Apparatus 20 iscapable of performing contact resistance measurements, stictionmeasurements, and/or other measurements over length scales in the micronand submicron ranges. In particular, the contact area defined byapparatus 20 can have an in-plane extent of microns to fractions of amicron. Apparatus 20 is also preferably capable of generating andmeasuring controlled applied forces and changes in applied forces in therange of microNewtons to tens of microNewtons.

[0024] Apparatus 20 comprises a fixed support 22, a thin film conductivesample 24 mounted on support 22, and a rounded-surface, conductivemovable upper member 26 capable of contacting sample 24. Conductivesample 24 is preferably a thin metal/alloy film having a thickness ofhundreds to thousands of Angstroms. Exemplary materials for sample 24include, without limitation, gold, chromium, molybdenum, ruthenium,iridium, indium tin oxide (ITO), platinum, palladium, or any otherconductive material to be evaluated. The thickness and surfaceproperties/treatment of sample 24 preferably correspond to the actualthickness and surface properties of at least one of the contact films tobe used in the microswitches of interest, such that the data measuredfor sample 24 is indicative of the properties of the film(s) to be usedin the microswitches of interest. Sample 24 can be a multilayer film,e.g. a bilayer film. Such a multilayer layer can include a conductiveouter layer, and an inner layer or layers formed by conductive orinsulative materials such as metals, polymers, or other desiredmaterials.

[0025] Apparatus 20 further comprises control/acquisition components 30electrically and mechanically connected to sample 24 and member 26.Control/acquisition components 30 include mechanical and electroniccontrol components for controlling the relative positions of sample 24and member 26. Control/acquisition components 30 further includeacquisition electronics for acquiring data indicative of contactresistance and stiction values between sample 24 and member 26. Theacquisition electronics preferably include a voltage or currentmeasurement unit for determining the current passing between sample 24and member 26 and/or other data indicative of the contact resistancebetween sample 24 and member 26. The acquisition electronics preferablyalso include a force setpoint indicator for measuring the applied forceapplied between sample 24 and member 26. In particular, the forcesetpoint indicator is used to record the force applied to member 26 at atimepoint during a retraction of member 26 when the current betweensample 24 and member 26 drops to zero, which will occur when the sample24 and the member 26 are no longer in contact, as described in furtherdetail below.

[0026] Support 22 preferably comprises a conventional fixed AFM chuck32, and a sample substrate 34 mounted on chuck 32. Substrate 34 ispreferably a silicon or glass substrate having a thickness of about 100to 1000 microns, on which conductive sample 24 is deposited. Substrate34 may include a conductive layer deposited onto a glass or siliconsupport, in contact with sample 24. Such a substrate conductive layermay be used to reduce the parasitic resistance of the measurementcircuit, as illustrated below. The upper surface of sample 24, oppositesubstrate 34, forms a contact surface for contacting upper member 26.

[0027] Upper member 26 comprises an elongated rigid cantilever arm 38,and a rounded end piece 40 mounted at the distal end of arm 38. Aconductive coating (sample) 44 extends over end piece 40 and arm 38, forproviding an electrical conduction path from the proximal end of arm 38to a contact surface defined along end piece 40. Conductive coating 44is preferably formed by a material of interest to be evaluated incontact resistance and/or stiction measurements to be performed withapparatus 20. Such a material may include, without limitation, gold,chromium, molybdenum, ruthenium, iridium, indium tin oxide (ITO),platinum, palladium, or any other conductive material to be evaluated.Coating 44 can be formed by a multilayer-film, as discussed above withreference to sample 24. Coating 44 can be a bi-layer including, forexample, an inner layer providing a low resistance to current flow, andan outer layer formed of the material of interest. The inner layer canserve to reduce the parasitic resistance of the measurement circuit.Coating 44 can be identical in composition to sample 24. Coating 44preferably has a thickness of hundreds to thousands of Angstroms.

[0028] The proximal end of arm 38 is connected to control/acquisitioncomponents 30. Control/acquisition components 30 control a linear,vertical motion of arm 38. Controlling the position and motion of arm 38allows controlling the contact force applied by end piece 40 to thecontact surface of conductive sample 24. The inner part of arm 38 ispreferably made of stainless steel or another rigid composition. Thegeneral shape (major axis) of arm 38 can make an acute angle, e.g. about10-15°, with the horizontal direction.

[0029] End piece 40 is preferably a smooth, spherical ball having adiameter of 10-100 μm, in particular about 40-60 μm, for example about50 μm. In general, balls having diameters or radii of curvature ofbetween as low as 1 μm and as high as 500 μm may also be used. Thegeometry of a sphere contacting a flat surface is well suited to usingthe Hertz equations of contact mechanics. The Hertz equations, incombination with contact resistance theory, can be used toquantitatively analyze the contact resistance properties of thematerials and switches of interest. Furthermore, a spherical (Hertzian)shape for end piece 40 can allow simplifying the theoreticalcharacterization of the contact surface. End piece 40 preferablycomprises an inner core 46, and the part of conductive coating 44disposed over inner core 46. Inner core 46 is preferably made of a hardmaterial such as tungsten, titanium, or silica. The part of conductivecoating 44 disposed over the lower part of inner core 46 defines arounded, preferably spherically-curved, contact surface 48 capable ofcontacting sample 24. As described-above, conductive coating 44preferably extends over a large enough area of upper member 26 so as toprovide an electrically-conductive path between contact surface 44 andthe acquisition electronics of control/acquisition components 30.

[0030] As is apparent, it is the shape of end piece 40 along contactsurface 48, rather than the overall size of end piece 40, that primarilydetermines the contact resistance characteristics of end piece 40.Consequently; end pieces that are not generally spherical or rounded,but have rounded (e.g. spherical, cylindrical, ellipsoidal, etc.)contact surfaces may be used. The relevant parameter for such an endpiece is the radius of curvature along the contact surface, rather thanthe overall size of the end piece. For a 60 μm spherical ball, theradius of curvature along the contact surface is 30 μm. For a rounded,non-spherical end piece, the radius of curvature along the contactsurface may vary within a range.

[0031] End piece 40 is preferably attached to arm 38 by a non-conductiveconventional epoxy. Alternatively, end piece 40 may be attached to arm38 using a conductive epoxy such as silver epoxy, other types ofadhesives, or mechanical fasteners. It was empirically observed thatcommercially available conductive epoxies such as silver epoxy havepoorer adhesive properties than available non-conductive epoxies. Suchconductive epoxies may also introduce relatively high resistances in theconductive path between the contact surface and the acquisitionelectronics. Moreover, core 46 and arm 38 may include native surfaceoxides, whose resistances may be significant even if the resistance of aconductive epoxy is relatively low. Thus, applying an externalconductive coating to member 26 may be desirable even if a conductiveepoxy were used to attach end piece 40 to arm 38.

[0032] To perform measurements of the contact resistance between lowersample 24 and upper sample 44, upper member 26 is lowered until endpiece 40 contacts sample 24. The position of end piece 40 is thencontrolled to apply desired forces to sample 24. Preferably, the appliedforces are between microNewtons to milliNewtons, and in particular onthe order of tens of microNewtons. The applied force is preferablycontrolled with an accuracy on the order of microNewtons to tens ofmicroNewtons. Generally, the applied forces are selected according tothe force expected to be applied to the microswitch of interest duringthe operation of the microswitch. For example, the applied forces can beselected to be in a range centered around an average applied forceexpected during the operation of the microswitch. The magnitude of theapplied force is measured using conventional AFM components ofcontrol/acquisition components 30. The acquisition electronics ofcontrol/acquisition components 30 are used to record electricalparameter (e.g. voltage or current) values indicative of the contactresistance between end piece 40 and sample 24 at various values ofapplied force between end piece 40 and sample 24. Contact resistancevalues are then computed from the recorded electrical parameter values.

[0033]FIG. 1-B shows a schematic illustration of an exemplary contactresistance detection circuit 50 according to an embodiment of thepresent invention. Circuit 50 includes several resistances arranged inseries outside of the acquisition electronics of apparatus 20, asdescribed below. A contact resistance 52 (Rc) is the resistanceintroduced by the interface between samples 24, 44. A first parasiticresistance 54 (R_(p1)) is the resistance of upper member 26 and theother part of the electrical path between contact surface 48 and theacquisition electronics. A second parasitic resistance 56 (R_(p2)) isthe resistance of sample 24 and the other part of the electrical pathbetween sample 24 and the acquisition electronics. The parasiticresistances 54 and 56 can be thought of as an equivalent, singleparasitic resistance R_(p) incorporating all the parasitic resistancesof circuit 50. In typical present applications, the parasitic resistanceof circuit 50 can vary from less than one ohm to several ohms. Theparasitic resistance can depend on film resistivity, thickness,structure (e.g. bilayer or multilayer), and other parameters.

[0034] The parasitic resistances 54, 56 can be determined empirically ina calibration measurement. Such a calibration measurement can be takenusing the system described above at a high applied force, or by usingexternal probes. The force used in the calibration measurement ispreferably at least about an order of magnitude higher than the forceused for the contact resistance measurements. For example, for contactresistance measurement forces in the range of 1-50 microNewtons, acalibration applied force of at least about 500 microNewtons ispreferably used. A cantilever arm having a high enough spring constantto permit the application of such forces is then used. A calibrationmeasurement can also be performed by using thick gold films along thecontact surface, or by using bi-layer films. A parasitic resistancecalibration measurement may not be needed for performing measurements ofchanges in contact resistance as a function of applied force, since theparasitic resistance of the circuit should not change with appliedforce. The calibration measurement is used to obtain an precise value ofthe resistances in the measurement circuit, including leads, equipmentinternal resistances, external resistances etc., since the presentinvention measures such small and accurate values of contact resistance.This high force is in the calibration measurement to essentially ensureperfect contact and, therefore, near zero contact resistance, at whichpoint isolation of the rest of the measurement circuit resistances canoccur.

[0035] Circuit 50 further includes several components which are part ofthe acquisition electronics of apparatus 20. A constant voltage source60 has one terminal connected to sample 24, and the other terminalconnected to ground. Voltage source 60 preferably generates a DCvoltage, preferably between 0 and 5 V. In a present implementation,voltage source 60 is a 3.5 V d.c. power supply. The internal resistanceR_(i) of voltage source 60 is shown at 62. In a present implementation,internal resistance 62 is 1100 Ω. A voltage meter 68 is connected acrossa high, known measurement resistance 64. Measurement resistance 64 isconnected between upper member 26 and the ground terminal of voltagesource 60. Measurement resistance 64 is preferably equal to theequivalent resistance of the rest of circuit 50.

[0036] For the measurement circuit shown in FIG. 2, the contactresistance R_(c) between sample 24 and sample 44 is $\begin{matrix}{{R_{c} = {{R_{m}\left( {\frac{V}{U} - \frac{R_{m} + R_{i}}{R_{m}}} \right)} - R_{p}}},} & \lbrack 1\rbrack\end{matrix}$

[0037] where U is the voltage measured by voltage meter 68, and R_(p) isthe total parasitic resistance R_(p)=R_(p1)+R_(p2). For the resistancevalues in the implementation described above, the resolution achievedwith circuit 50 is on the order of a few ohms. The sensitivity of themeasured voltage U across R_(m) is highest when R_(m) is matched (orequal) to R_(i), assuming R_(i)>>(R_(c)+R_(p)). Improved resolutions canbe achieved by using different measurement circuits, such as for examplea current-measuring circuit including a logarithmic current amplifier.

[0038]FIG. 2 illustrates exemplary contact resistance data taken usingthe apparatus described for one Au—Au and two Au—Cr contact surfaces,and computed contact resistance data generated using a tunneling model.Curves 220, 222 and 224 show the measured Au—Au, Au—Cr, and Au-etched Crdata, respectively. The star-shaped data points illustrated at 226 werecomputed using electron tunneling theory assuming a work function thatvaries with applied force, to enable a match to the data. Thesquare-shaped data points shown at 228 were computed for a fixed workfunction and fixed film thickness. A gold-coated tip and gold-coatedsilicon substrate were used for the gold-on-gold measurements. Agold-coated tip and chromium-coated silicon substrate were used for thegold-on-chromium measurements. The gold films were deposited asdescribed in detail below. A physical vapor deposition (PVD) process wasused to deposit the two chromium films.

[0039] No oxide formed on the gold films. Consequently, the contactresistance at the Au—Au film interface was relatively low andindependent of applied force. The Cr film corresponding to the curvelabeled “Au-etched Cr” was etched after deposition by dipping into anacetic acid/hydrogen peroxide mix. The etching process resulted in theformation of a relatively thick (about 2 nm) oxide layer on the Cr filmsurface. Consequently, the contact resistance as a function of appliedforce stayed relatively high, and decreased only for applied forcevalues above 20 μN. The Cr film corresponding to the curve labeled“Au—Cr” was exposed to air but not etched, and had a thinner oxide film.The contact resistance for the Au—Cr interface decreased rapidly forapplied forces under 10-20 μN.

[0040]FIG. 3-A shows computed contact resistance data 320 as a functionof applied load for a Ru—Mo interface, with RuOx and MoOx contamination(oxide films) on the metal surfaces. FIG. 3-B shows computed contactresistance data as a function of applied load for a Ru—Mo interfacehaving no contamination and RuOx contamination only, respectively. Curve322 shows data for no surface contamination, while curve 324 shows datafor RuOx contamination only. The data was computed for a contact surfacedefined at the interface between a sphere and a planar film, using theHertz equations and the theory of contact resistance. For furtherinformation on contact resistance theory see for example R. Holm,“Electrical Contacts: Theory and Application,” New York,Springer-Verlag, 1967.

[0041] Referring back to FIG. 1-A, apparatus 20 can also be used toperform measurements of electrically-dependent (current-dependent)contact stiction forces on samples of interest. When two surfaces comeinto intimate contact, the two surfaces can become attached to eachother. The stiction phenomenon can be thought of as microwelding or coldwelding caused by formation of bonds between atoms along the twosurfaces. The stiction force between the two samples is the forcerequired to separate the samples after the samples are brought intocontact using an applied force. The microwelding phenomenon and theassociated stiction force can depend on the passage of current-throughthe contact surface. The passage of electrical current can causelocalized heating of the samples along the contact surface, thusenhancing the microwelding process and increasing the stiction force.For example, such localized heating can lead to melting of samplemicroasperities along the contact surface.

[0042] To perform a measurement of a current-dependent stiction force,upper member 26 is lowered until ball 40 contacts sample 24 and appliesa desired level of downward force to sample 24. A desired stictionmodulation voltage is applied using voltage source 60, such that acontrolled amount of current passes through the contact interfacebetween samples 24, 44. The stiction modulation voltage is preferablychosen such that the current passing through the contact interfacebetween samples 24, 44 is equal to or similar in magnitude (e.g. withina factor of ten) of the current expected during an operation of amicroswitch of interest. The current is preferably between 5 μA and 5mA, for example between 5 μA and 100 μA. Higher currents, on the orderof mA, may be used to simulate short-circuits or other atypicaloperating conditions that may occur for example if the switch issubjected to shocks. In a present implementation using the detectioncircuit 1-B, the stiction modulation voltage applied using voltagesource 60 is about 5 V.

[0043] An increasing upward force is then applied to upper member 26. Ifdesired, to prevent arcing between samples 24, 44 when samples 24, 44become separated, the voltage applied by voltage source 60 can bereduced to a stiction-measurement value before the upward force isapplied to upper member 26. The stiction measurement value is lower thanthe stiction modulation value, for example at least an order ofmagnitude lower than the stiction modulation value. In a presentimplementation, the voltage applied using voltage source 60 is reducedfrom a stiction modulation value of about 5 V to a stiction measurementvalue of about 0.2 to 0.3 V.

[0044] The force required to separate member 26 from sample 24 (thestiction force) is then recorded. The stiction force is preferablymeasured by recording the force corresponding to a cessation of currentflow between sample 24 and upper member 26, as detected by voltage meter68. Alternatively, the stiction force can be measured using conventionalmechanical (e.g. piezoelectric) components, in a manner which does notdepend on the passage of current through the contact interface betweensamples 24, 44.

[0045] The applied force values for stiction measurements can be (butneed not be) significantly larger than the applied force values forcontact resistance measurements. As an illustration, for evaluating amicroswitch such as a fingerprint sensor microswitch, the applied forcesof interest can be forces due to shocks or impacts applied to themicroswitch. If such an applied force leads to a stiction force higherthan the restoring elastic force of the microswitch, the switch canremain permanently closed regardless of whether an external force isapplied subsequently. In designing such a microswitch, the stictionforces resulting from expected shocks to the microswitch are ideallylower than the restoring force of the microswitch. In a presentimplementation, stiction force values are measured for applied forces oftens to hundreds of microNewtons. Depending on the application, suitableapplied forces may range up to tens of microNewtons, 1 mN, or hundreds(e.g. 200) of milliNewtons. In general, the applied force for thestiction force measurements can be selected according to expected impactapplied forces during the operation of the microswitch, for example in arange up to the maximum expected operating impact force. The appliedforces and voltages for the stiction force measurements can also beselected according to the expected normal operating conditions of theswitch of interest, as described above with reference to contactresistance measurements.

[0046]FIG. 4 shows measured stiction force data illustrating thedependence of stiction force on applied force for two Au—Au contacts: acontact between two unetched Au surfaces, and a contact between anunetched Au surface and an etched surface. The data points representedby the squares 420 are for the etched surfaces, while the data pointsrepresented by the circles 422 are for the unetched contact surface. TheAu films were sputtered from pure targets in a magnetron sputteringsystem. The unetched surfaces were as-deposited. The etched surface wasetched in an acetic acid/hydrogen peroxide mixture. Etching cleaned theAu surface and made it more prone to stiction, as shown. Cleaning the Ausurface of organic and other contaminants by etching can facilitatemicrowelding, and thus increase the stiction force required for a givenapplied force.

[0047] Apparatus 20 is preferably made by modifying a conventional,commercially-available atomic force microscope (AFM) as described below.A conventional sharp-tip AFM cantilever is replaced with a cantileverstructure having a flat or rounded contact surface in electricalcommunication with the acquisition electronics of the apparatus. Thecantilever structure can be kept connected to the acquisitionelectronics, rather than grounded. A voltage or current meter can beconnected to the chuck and arm as to as measure a voltage or currentindicative of the contact-resistance values of interest. For thepreferred design shown in FIG. 1-A, the hard inner core of spherical endpiece 40 is preferably epoxied to the inner part of arm 38 using anon-conductive epoxy. A suitable cantilever having an epoxied ball atits end can also be obtained commercially, for example fromVeeco/Digital Instruments, Santa Barbara, Calif. The resulting assemblyis mounted into a vacuum system, for depositing conductive coating 44over the assembly. Conductive coating 44 can be deposited by ion beamdeposition, sputtering (physical vapor deposition), chemical vapordeposition (CVD), electroplating, dip-coating, or other such methods.

[0048] In a present implementation, coating 44 is deposited by ion beamdeposition. FIGS. 5-A and 5-B illustrate schematically the preferredassembly mounting geometry employed for the deposition of coating 44 byion beam deposition. FIG. 5-A shows member 26 before the deposition ofcoating 44, while FIG. 5-B illustrates member 26 after the deposition ofcoating 44. The arrows 80 denote the direction of motion of the coatingmaterial toward member 26. The interface area along which end piece 40and arm 38 form a most acute angle is shown at 82. As shown in FIGS.5-A-B, end piece 40 can be attached to arm 38 along a major longitudinalsurface of arm 38, rather than along the transverse end surface of arm38.

[0049] As illustrated, upper member 26 is mounted within the depositionvacuum system such that end piece 40 does not shield the acute-angleinterface area 82 from the source of coating material. Allowing endpiece 40 to shield interface area 82 can lead to a suboptimal electricalconnection between the coating area covering end-piece 40 and thecoating area covering arm 38. Preferably, upper member 26 is positionedsuch that interface area 82 squarely faces the source of coatingmaterial. Such a positioning corresponds to an acute angle between thecoating deposition direction illustrated by arrows 80 and the surface ofarm 38 facing the source of coating material. In a presentimplementation, the acute angle is preferably between 30° and 60°, inparticular about 45°.

[0050] The above-described systems and methods allow performing contactresistance and stiction measurements under conditions similar to thosein microswitches suitable for use in fingerprint recognition microswitcharrays, or other microswitches subject to similar forces and operatingconditions. Although AFM devices have found multiple applications,conventional AFM devices are typically used to profile surfaces withhigh resolution. For such applications, typical AFM tips are very sharpand are operated under ultra-low force conditions. Conventional AFM tipscommonly end in a sharp point having a size of tens to hundreds ofnanometers. Many developments in AFM have focused on improving thesharpness of the AFM tip, in order to improve the resolution of thedevice as it scans surfaces of interest. Such ultra-sharp tips areordinarily not suitable for performing contact resistance or stictionmeasurements for typical microswitches. Moreover, typical AFM tips aremade of insulating materials such as diamond, silicon, or siliconnitride. Consequently, a conventional AFM device is preferably modifiedas described above in order to perform contact resistance and/orcurrent-dependent stiction measurements.

[0051] AFM devices have also been used to characterize electricalproperties of samples of interest. In U.S. Pat. No. 6,208,151, Thomas etal. describe an AFM having a conductive sharp tip for mapping voltagegradients in a planar sample. Current is injected from the tip into thesample, and voltages in the sample plane are measured using a voltmeter.The contact resistance between the tip and the sample can be a nuisancein the measurements described by Thomas et al., and its effects arecompensated for by injecting constant current into the sample. In U.S.Pat. No. 5,723,981, Hellemans et al. describe employing a sharp tip formeasuring voltages across a biased semiconductor device. Applying a highforce for the measurement minimizes the effect of any contact resistanceon the voltage measurements. In their article “Lateral and VerticalDopant Profiling in Semiconductors by Atomic Force Microscopy usingConducting Tips,” J. Vac. Sci. Tech. A, 13(3):1699-1704 , May-June,1995, De Wolf et al. describe a method of determining the spatialdistribution of charge carriers in semiconducting structures using anatomic force microscope. A sharp conductive tip is scanned across asample, and the local spreading resistance of the sample is determinedfrom the measured data. The references cited above focus on determiningelectrical properties within a single sample, rather than measuringcontact resistances or stiction force values between two samplesurfaces.

[0052] Bulk (millimeter-size) measurement devices are not normallycapable of controlling the applied force finely enough to allow adequatesimulation of expected microswitch operating conditions. Bulkmeasurement devices may only be capable of applying relatively highforces, and may not be useful in controlling small changes in theapplied force. In addition, bulk measurement devices havingmillimeter-size radii of curvature along the contact surface can belimited in their spatial resolution.

[0053] It is apparent that the above embodiments may be altered in manyways without departing from the scope of the invention. For example, anend piece having a cylindrical, ellipsoidal, or other rounded (e.g.non-spherical) contact surface may be used. A curved, non-spherical endpiece may be used while maintaining the radius of curvature of the endpiece within a desired range along the contact surface. An intermediateconductive structure of surface can be inserted between two electrodesof interest in order to measure a contact resistance of interest. Forexample, an intermediate gold structure may be placed in contact with asurface of interest (e.g. a chromium-oxide surface), and a second goldelectrode may be brought into contact with the gold structure. It isunderstood that other dimensions and materials can be suitable for usewith,the present invention. Further, various aspects of a particularembodiment may contain patentably subject matter without regard to otheraspects of the same embodiment. Still further, various aspects ofdifferent embodiments can be combined together. Accordingly, the scopeof the invention should be determined by the following claims and theirlegal equivalents.

1. An atomic force microscope arm for performing atomic force microscopymeasurements of an electrically-dependent property of a sample,comprising: an elongated support member having a proximal region and adistal region; a rounded piece mounted on the support member along thedistal region, the rounded piece defining a contact region; and aconducting film continuously disposed over the contact region and thesupport member up to the distal region, for electrically connecting thatportion of the conducting film disposed over the contact region to thedistal region of the elongated support member.
 2. The apparatusaccording to claim 1 wherein the rounded piece at the contact region hasa radius of curvature higher than 10 μm and lower than 100 μm.
 3. Theapparatus according to claim 2 wherein the rounded piece has asubstantially spherical shape along the contact region.
 4. A method ofmaking an atomic force microscope arm for performing atomic forcemicroscopy measurements of an electrically-dependent property of asample, comprising: establishing an elongated support member having aproximal region and a distal region; mounting a rounded piece mounted onthe support member along the distal region, the rounded piece defining acontact region; and coating the rounded piece and the support memberwith a conducting film continuously over the contact region and thesupport member up to the distal region, for electrically connecting theconducting film disposed over the contact region to the distal region.5. The method according to claim 4 wherein the rounded piece at thecontact region has a radius of curvature higher than 10 μm and lowerthan 100 μm.
 6. An atomic force microscope apparatus for performingtesting comprising: a sample holder for holding a first planar thinfilm; an arm comprising a rounded end piece including a second roundedthin film wherein a radius of curvature of the rounded piece existsalong a contact surface between the first film and the second film, andwherein the arm and the sample holder are capable of relative movementtherebetween to establish a contact between the first thin film and thesecond thin film; and a measurement circuit electrically connected tothe first thin film and the second thin film, for making a measurementrelated (need to reword) a characteristic related to the first film andthe second film contacting.
 7. The apparatus according to claim 6wherein the measurement circuit is a contact resistance measurementcircuit for making a measurement of a contact resistance between thefirst thin film and the second thin film when the contact between thefirst thin film and the second thin film is established.
 8. Theapparatus according to claim 6 wherein the radius of curvature is higherthan 10 μm and lower than 100 μm.
 9. The apparatus of claim 7 whereinthe contact between the first thin film and the second thin film isestablished with a first force not exceeding 1 mN.
 10. The apparatusaccording to claim 6 wherein the measurement circuit is a stiction forcemeasurement circuit for making a measurement of a current-dependentstiction force between the first thin film and the second thin filmafter the contact between the first thin film and the second thin filmis established.
 11. The apparatus of claim 10 wherein the contactbetween the first thin film and the second thin film is established at afirst force not exceeding 1 N.